The present invention relates to a ZnS-series sintered material and a method for producing the same, a sputtering target formed from the sintered material, a thin film, and an optical recording medium having the thin film.
A ZnS-series material has been well known not only as a fluorescent substance but also as an electroluminescent substance. In the field of photoelectronics, the ZnS-series material is used as a thin film that has light transmission properties and a high refractive index. For example, in a phase change-type optical recording medium comprising a recording layer composed of an alloy of tellurium (Te) or antimony (Sb), the ZnS-series thin film is used as a protecting layer for protecting the recording layer. This medium has been used for rewritable optical disks, such as rewritable compact disks (CD-RW), random access memory digital video disks (DVD-RAM), DVD-rewritable (DVD-RW), DVD+rewritable (DVD+RW), and the like.
FIG. 4 shows the basic structure of an optical disk that serves as an optical recording medium. A first protecting layer 2 (21) is formed on a disk substrate 10. A recording layer 3 is formed on the first protecting layer 2 (21), and further, a second protecting layer 2 (22) is formed. A reflecting layer 4 is formed on the second protecting layer 2 (22). The reflecting layer 4 is composed of aluminum, gold, silver or an alloy containing these metals as a main component.
A laser beam is irradiated on the disk substrate 10 of the above-mentioned optical disk. The laser beam is transmitted through the protecting layers 2 (21, 22) to the recording layer 3 and is reflected by the reflecting layer 4. The reflected laser beam returns to the side of the irradiation source through the recording layer 3 and the protecting layers 2 (21, 22). In the phase change-type optical recording medium, in recording, the laser beam, which is modulated according to the signal strength, is irradiated to the optical recording medium. The heat energy of the laser beams causes a phase change in the recording layer. For example, the alloy thin film in the recording layer undergoes an alternate change between the crystal phase and the amorphous phase. This phase change is recorded as a signal. In reproduction, a laser beam is irradiated, causing a phase change in the recording layer 3. The change in reflection intensity of the laser beam in accordance with the phase change of the recording layer 3 is detected as a signal.
The protecting layers 2 transmit a laser beam and protect the recording layer 3 by contacting both surfaces of the recording layer 3. The protecting layers 2 are composed of, for example, a ZnS element or a ZnSxe2x80x94SiO2 composite.
In optical disks that are rewritable on demand, during recording/erasing of the signal information by laser beam irradiation, the above-mentioned protecting layers 2 (21, 22) are heated in a temperature range of from 400 to 700xc2x0 C. though for a very short time. Then, the protecting layers 2 (21, 22) undergo a considerable temperature change. Therefore, ZnS, which has an excellent heat resistance, has been used in the protecting layers 2 (21, 22). However, ZnS has a problem that a grain growth occurs due to repeated laser beam heating. A ZnSxe2x80x94SiO2 composite is a material obtained by adding SiO2 to ZnS. For example, a ZnSxe2x80x94SiO2 composite having the composition of 80% by mole of ZnS and 20% by mole SiO2 is known. The addition of SiO2 suppresses grain growth caused by the repeated heating. Thus, the SiO2-series thin film of the composition of 80% by mole ZnS and 20% by mole SiO2 has been prepared to have a fine structure such that the particle diameter of the crystal is small.
Further, due to the high power laser beam irradiation during writing, the recording layer 3 undergoes a temperature change. That is, the recording layer 3 is heated and cooled. The two protecting layers 2 (21, 22) directly contact the recording layer 3. For preventing the protecting layers 2 (21,22) from reacting with the recording layer 3, they must have a low chemical reactivity to the alloy used for the recording layer 3 in a temperature range from room temperature up to a maximum of 700xc2x0 C.
The protecting layers 2 (21, 22) can be deposited in a form of thin film by a radio frequency (RF) sputtering process. According to this process, the disk substrate 10 and a target are arranged to face each other in an RF sputtering apparatus. A ZnSxe2x80x94SiO2 sintered material is used as the target material. Then, a high frequency plasma is generated in a high vacuum and rare argon (Ar). Argon ions, which are generated, cause the target to release material, which forms a thin film (the protecting layer 2) of the material on the disk substrate 10. In addition, after the deposition of the recording layer 3, the protecting layer 2, which is a thin film, is deposited on the recording layer 3 in the RF sputtering apparatus. Examples of ZnSxe2x80x94SiO2 sintered materials are disclosed in Japanese Unexamined Patent Publication Nos. Hei 11-278936 and 7-138071.
To produce a sintered material, for example, a method in which a mixed powder of ZnS and silica is subjected to hot pressing in an inert gas atmosphere at a specific high sintering temperature and a method in which a shaped body comprising a mixed powder is subjected to atmospheric sintering have been used. Further, a hot isostatic pressing (HIP) method for further rendering the sintered material dense has also been used.
Conventionally, to deposit the protecting layer 2 in the above-mentioned optical recording medium using these ZnSxe2x80x94SiO2 sintered materials, only the RF sputtering process can be employed. This is because a direct current (DC) sputtering process cannot be employed due to the high electric resistance of the ZnSxe2x80x94SiO2 sintered material. However, in the RF sputtering process, it is difficult to apply a high electric power to the target. For this reason, the sputtering rate and the efficiency of deposition are lowered, and the productivity of the thin film for the optical recording medium cannot be improved.
In RF sputtering, high frequency electrical heating (high frequency heating) is generated in the disk substrate 10, which is made of a polymer such as polycarbonate. This may cause thermal damage to the disk substrate 10. This is also disadvantageous in the productivity of the optical recording medium. In the production of large capacity optical disks, it is necessary that the deposition rate be increased to improve the productivity of the thin film. For this reason, it has been preferred that a deposition process other than RF sputtering be applied to the production of optical disks.
To control the uniformity and thickness of the protecting layer 2, it is preferred to use a target having a size corresponding to the size of disk substrate 10. However, it is difficult to obtain a large, dense ZnSxe2x80x94SiO2 sintered material. Therefore, both the sintering strength and the production efficiency are low when a large piece of ZnSxe2x80x94SiO2 is used.
The target sintered material is required to have a small number of internal pores and a high relative density. Since it is difficult to obtain a large, dense piece of ZnSxe2x80x94SiO2 sintered material, the porosity is relatively large. When sputtering is performed using a target sintered material having a large porosity, air contained in the sintered material is released, and to maintain the atmosphere in the sputtering apparatus at a high vacuum level during sputtering is difficult. When a composite material is deposited without maintaining the atmosphere in a high vacuum, the composition of the resulting thin film may become different from that of the sintered material. Further, when the atmosphere is not maintained in a high vacuum, the sputtering rate is lowered, which lowers productivity.
Although the thin film of a ZnS element has a relatively high refractive index due to addition of SiO2 thereto, the refractive index of the ZnSxe2x80x94SiO2-series thin film is lower than that of the ZnS element thin film. When the refractive index of the protecting layers 2 (21, 22) composed of a ZnSxe2x80x94SiO2-series thin film is too low, as compared to the refractive index of each of the crystal phase and the amorphous phase of the recording layer 3 composed of the Gexe2x80x94Texe2x80x94Sb alloy layer, the optimal thickness of the protecting layers 2 (21, 22) for optimizing the reproduction signal strength ratio by a laser signal cannot be reduced significantly. When the thickness of the protecting layers 2 is relatively great, the heat caused by the laser beam irradiation in the protecting layers 2 (21, 22) during writing is difficult to remove due to low heat conductivity of the ZnSxe2x80x94SiO2-series thin film. Thus, not only does the temperature of the protecting layers 2 rise, but the protecting layers 2 are easily peeled off.
A large capacity optical disk requires an optical recording medium having a high recording density. For increasing the density, attempts have been made to increase the revolution speed of the disk and shorten the wavelength of the laser beam. In the case where light having a wavelength of 400 nm is used, the rate change in the complex index of refraction of the recording layer 3 (composed of, for example, a Gexe2x80x94Texe2x80x94Sb alloy layer) following the phase change is small, and thus, the S/N ratio of the signal is small, as compared to the case where a light having a wavelength of 830 nm or 780 nm is used. To prevent the lower S/N ratio when the shorter wavelength is used, the protecting layers 2 (21, 22) are also required to have a higher refractive index. The ZnS element thin film has a refractive index n of about 2.35 for light having a wavelength of 400 nm, whereas the ZnSxe2x80x94SiO2-series thin film has a lower refractive index n due to addition of SiO2. However, preferably, the refractive index should be 2.50 or more.
As for the phase change-type optical recording medium, the substrate 10 of a conventional CD has a thickness of 1.2 mm, whereas the substrate 10 of a DVD has a thickness as small as 0.6 mm. The ZnSxe2x80x94SiO2-series thin film not only has a low refractive index but also a low heat conductivity, so that internal stress is likely to occur in the thin film. Therefore, residual stress is generated in the substrate 10 and the optical recording medium. The stress may reduce the reliability of the optical recording medium.
As mentioned above, if it were possible to obtain a stable composite material having optical characteristics of a low electric resistance and a high refractive index, as compared to the conventional ZnSxe2x80x94SiO2 composite material, such a material would be useful for the protecting layers 2 (21, 22) used in the optical recording medium. Such a material is expected to improve the performance of the optical recording medium, improve the efficiency of the production process, and reduce the production costs.
It is an object of the present invention to provide a sintered material having a low electric resistance and an excellent shaping property.
It is another object of the present invention to provide a sintered material composed of a ZnS-series material which can be used as a target for direct current sputtering.
It is another object of the present invention to provide a raw material powder for a sintered material.
It is another object of the present invention to provide a method for producing a sintered material.
It is another object of the present invention to provide a thin film having a refractive index higher than that of a thin film composed of a ZnS element and an optical recording medium having the thin film.
In one aspect of the invention, a ZnS-series sintered material includes ZnS as a main component and niobium oxide.
In another aspect of the invention, a sputtering target is made of a ZnS-series sintered material which includes ZnS as a main component and niobium oxide.
In yet another aspect of the invention, a raw material powder for forming a ZnS-series sintered material is provided. The raw material includes a ZnS powder having an average particle diameter of 0.5 to 20 xcexcm and a niobium oxide powder having an average particle diameter of 5 xcexcm or less.
In another aspect of the invention, a method for producing a ZnS-series sintered material includes preparing a mixture of a ZnS powder having an average particle diameter of 0.5 to 20 xcexcm and a niobium oxide powder having an average particle diameter of 5 xcexcm or less; and hot pressing the mixture at a temperature of 800 to 1100xc2x0 C. to obtain a sintered material.
In yet another aspect of the invention, a method for producing a ZnS-series sintered material includes shaping a mixture of a ZnS powder having an average particle diameter of 0.5 to 20 xcexcm and a niobium oxide powder having an average particle diameter of 5 xcexcm or less into a predetermined shape; and sintering the resultant shaped body in an inert gas at a temperature of 700 to 1200xc2x0 C., to obtain a sintered material.
In further aspect of the invention, a ZnS-series thin film includes ZnS as a main component and niobium oxide.
In another aspect of the invention, a method for producing a ZnS-series thin film includes preparing a ZnS-series sintered material comprising ZnS as a main component and niobium oxide; and subjecting the ZnS-series sintered material to direct current suputtering to form a ZnS-series thin film.
In yet another aspect of the invention, an optical recording medium includes a recording layer for recording a signal of a laser beam irradiation as a phase change; and a protecting layer for protecting the recording layer by coating. The protecting layer is made of ZnS-series thin film which includes ZnS as a main component and niobium oxide.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.